Impact of the COVID-19 lockdowns on the risk from legionella bacteria

Importance of health and safety and wellbeing of the staff, visitors and the public has been a focus and priority of many employers and building owners. Even more now, when we all are being affected by the pandemic. When tackling the spread of the virus, it is easy to lose focus on our general day to day obligations.

As our work patterns, movements and behaviour change, this can have a significant impact on ​aspects of the safety of our properties, and this includes the risk associated with legionella bacteria. Health and Safety Law continues to apply, and duty holders still maintain a legal obligation to protect those in and around their properties from exposure to legionella bacteria both now and in the future.

Legionella bacteria can grow at temperatures between 20°C and 45°C, where a suitable nutrient is available and where the turnover of water is low. As businesses increasingly move to a home-working model, the demand for water in many commercial buildings will be significantly reduced. As water usage is reduced the risk of low turnover and stagnation increases.

Hot and cold water systems

The main risk to hot and cold water systems caused by the ongoing coronavirus pandemic is the reduction in demand for water. Water usage should be closely monitored during this period, and the volume of stored water in cold water storage tanks should be reduced where possible, to ensure complete daily turnover is maintained.

Weekly flushing regimes must be reviewed and continued where possible, to ensure that all water services which are infrequently used - are included. For many buildings, this may include all facilities.

Reduced turnover of water can lead to increased temperatures in cold water systems. This can be particularly significant in large, high rise buildings and complex water systems. Temperature monitoring should continue and, where issues are identified, the frequency of flushing and temperature monitoring should be increased as necessary to maintain cold water systems below 20°C.

It is important that all routine maintenance tasks advised by your assessment continue in line with ACoP L8 and HSG274 Part 2. In multi-occupancy buildings, tenants should be contacted to ensure they continue to fulfil their obligations with regard to legionella control, including at least weekly flushing of all infrequently used water services.

However, as the government implemented strict new measures to tackle the spread of coronavirus, it may be difficult to fulfil all maintenance and monitoring tasks. We advise to implement additional control measures; these may include locking showers or removing shower heads to prevent the aerosol generation, additional flushing without the release of aerosols and increased microbiological sampling. When all of the above cannot be completed, you should contact your Legionella Services provider for advice on how to safely shut down and re-start the system when we are ready to resume the normal life.

HSG 274 part 2 gives a detailed instruction of how to safely shut down the system and what needs to be done when it’s time to reopen.

Buildings temporarily taken out of use (mothballing)

  • Where a building, part of a building or a water system is taken out of use (sometimes referred to as mothballing), it should be managed so that microbial growth, including legionella in the water, is appropriately controlled.

  • All mothballing procedures are a compromise between adequate control of microbial growth, the use of water for flushing (while avoiding waste) and degradation of the system by any disinfectant added. Where disinfectants are used, these should leave the system fit for its intended purpose.

  • In general, systems are normally left filled with water for mothballing and not drained down as moisture will remain within the system enabling biofilm to develop where there are pockets of water or high humidity. The water in the system also helps to avoid other problems associated with systems drying out, including failure of tank joints and corrosion in metal pipework. The systems should be recommissioned as though they were new (ie thoroughly flushed, cleaned and disinfected) before returned to use.

Recommissioning Water Systems

It is essential that when buildings reopen following the lifting of COVID-19 restrictions, that any water system is not simply put straight back into use. During the period of shutdown it would be sensible to formulate a recommissioning plan for each water system to allow safe start-up and assurance to users that it is safe. Duty holders are likely to be able to access competent help from service providers remotely during the period of restricted movement.


Any plan for recommissioning buildings must take into account the safety of the operatives carrying out the work. It is foreseeable that the hazard present within water systems in this situation would be greater than normally expected. Reasonably practicable measures such as limiting aerosol, minimising exposure and use of RPE should be considered.


The minimum expectation for small, simple hot and cold water systems would be flushing through with fresh mains water. Larger buildings, those with tanks, showers, calorifiers and more complex pipework the expectation is likely to be for more extensive flushing followed by cleaning and disinfection.


In all cases where systems are being recommissioned it is sensible to have evidence to prove/reassure that the recommissioning process has been effective. Sampling to BS7592 should be considered for recommissioning plans to validate the effectiveness of the process. As per HSG274 part 2, samples should be taken 2-7 days following recommissioning and not on the day of disinfection. Follow up samples may need to be considered as part of the recommissioning plan.


If you require further information or advice regarding legionella control or have concerns specific to maintaining the risks associated with legionella bacteria during the coronavirus pandemic contact us

Stay Safe

Sylwia Leszkiewicz MWSOC

Director & Senior Legionella Consultant

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The First Principle Group Ltd

Coventry | West Midlands | England 

Info@thefirstprinciple.co.uk

T: 02476880450

M: 07896049085

Company Registration Number: 10754100

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